1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus.
2. Description of the Related Art
In a motor-driven power steering apparatus, as described in Japanese Patent Application Laid-Open No. 2000-318627 (patent document 1), there is a structure in which rotation of an electric motor is converted into a linear stroke of a rack shaft by a power transmission mechanism. A brushless motor is used as the electric motor during steering assist of a tire wheel connected to the rack shaft. Coil winding is wound around a core slot of a stator core, driving current is supplied to the coil winding by a motor drive means, and the electric motor controls the apparatus to generate a predetermined steering assist force.
A control system of the electric motor has a steering torque sensor detecting a steering torque which the driver inputs to the steering wheel, a vehicle speed sensor detecting a speed of the vehicle, a current sensor detecting a motor current actually flowing through the electric motor, and a rotational position sensor detecting a rotational position of a rotor of the electric motor.
When the steering torque sensor detects the steering torque, a target current is supplied to the electric motor such that the steering torque matches to a target steering torque, and steering operation is assisted on the basis of an output torque of the electric motor. The target steering torque is compensated in correspondence to the vehicle speed detected by the vehicle speed sensor. When supplying the target current to the electric motor, the motor current actually flowing through the electric motor is detected by the current sensor, and a feedback control acts such that a deviation between the actual current and the target current is not generated.
The electric motor has a brushless motor, a stator having plural phases of coil windings, and a rotor having a permanent magnet. It is necessary to sequentially switch the supply of the driving current to exciting phases of the respective coil windings in correspondence to angle of rotation of the rotor (position of magnetic poles of the permanent magnet). Accordingly, the rotational position of the rotor with respect to the stator is detected by the rotation angle sensor.
In the conventional motor-driven power steering apparatus, if abnormality of the sensor is detected in the control system of the electric motor, an assist stop control stopping all feeding to the electric motor is executed. In an assist stop control state, the assist of the electric motor is not applied to the steering system which the driver executes the steering from the input shaft via the rack shaft. Motor friction is applied as a load at the same time when the assist of the electric motor is not generated, and the steering torque of the driver becomes too large.